The Duke of Edinburgh spent a third night in a London hospital after being admitted as “a precautionary measure” and has yet to receive any visitors. Prince Philip, 99, husband of Queen Elizabeth II, was taken to the private King Edward VII Hospital on Tuesday evening. Buckingham Palace said he was admitted as a precaution after feeling unwell and remained in hospital for “observation and rest”. His illness is not believed to be related to Covid-19. Both the queen, 94, and Prince Philip received a first dose of a vaccine against coronavirus in early January. Hospital visits were already limited because of the pandemic and royal enthusiasts suggested Prince Philip's isolation was a good sign for his recovery. Penny Junor, who has written books about the British royal family, said she was "very optimistic" that he would recover. She told the BBC that if there were serious fears for his health "the queen would definitely find a way of getting there, and probably Prince Charles as well". “He has eaten well, has not been a smoker for years and is not a heavy drinker. He is very active in body and mind and always has been,” she said. Prince Philip, who retired from public duties in 2017, rarely appears in public. His most recent event was a military ceremony at Buckingham Palace in July. He has been staying at Windsor Castle with the queen during England's lockdown. The royal family has continued its duties while Prince Philip is in hospital, with the queen speaking to Royal Navy Admiral Tony Radakin from Windsor, while Prince Charles and the Duchess of Cornwall visited a hospital in Birmingham on Wednesday. The royal household is planning celebrations to mark Prince Philip's 100th birthday on June 10, Covid-19 restrictions permitting. Prince Philip and the queen have been married since 1947 and is the longest-serving royal consort in British history. He and the queen have four children, eight grandchildren and nine great-grandchildren. He last had a spell in hospital in December 2019 when he spent four nights at the King Edward VII Hospital. He was treated for an underlying health condition and was discharged on Christmas Eve.